This invention relates to electromechanical transducers and more particularly to such transducer assemblies employing piezoresistive semiconductor strain gages on thin diaphragms which are secured to a housing by electrostatic glass bonds.
The invention further relates to techniques for fabricating an improved transducer assembly employing glass bonds.
There is a class of pressure transducers which utilize the piezoresistive effect to enable high outputs. These units are semiconductors and have resulted in the construction of electromechanical force transducers with superior output characteristics and operating frequencies as compared to those of the prior art.
The piezoresistive transducer technology is compatible in many respects with integrated circuit techniques.
Generally, a transducer comprises a relatively thin diaphragm which may be constructed of silicon. A strain gage bridge assembly is then diffused or grown onto the silicon diaphragm as are suitable contact arrangements. The diaphragm plus the gages and contacts are then mounted to a suitable housing in order to properly protect and utilize the transducer assembly.
It is desirable to secure the diaphragm to the housing so that a hermetic seal is obtained. This assures protection of the gages and enables accurate and reliable measurements while providing better high temperature operation.
The prior art shows various schemes for formulating a bond between the diaphragm and the housing. Thus, epoxy bonds are used. These bonds suffered as the epoxy exhibited plastic behavior at raised temperatures causing spurious forces to be applied to the gage via the diaphragm.
Glass bonds were also used and are known. One could secure the diaphragm to a housing be means of solder glass. These glasses devitrify at a temperature compatible with methods of lead attachment to silicon. Upon devitrification of these glasses, a partially crystalline structure develops which results in a seal much stronger and harder than available with a vitreous glass. The difficulty with these techniques is that the bond was not uniform and the sealing process is not compatible with certain fabrication processes employed in the fabrication of piezoresistive sensors. Another problem was that the gages were subjected to a compressive force during the cooling process.
The prior art recognized that strong hermetic seals could be acheived between glass and various metals at relatively low temperatures by applying an electrostatic bias across the glass to the metal interface during the seal process. Such seals were achieved at temperatures between 200.degree.F to 400.degree.F below the melting point of the glass. In any event, this technique still presented problems when attempting to fabricate high quality transducers.
For example, U.S. Pat. No. 3,654,579 entitled "ELECTROMECHANICAL TRANSDUCERS AND HOUSING", issued on Apr. 4, 1972 to A. D. Kurtz, et. al., and assigned to the same assignee herein, shows an improved transducer. The transducer employs a semiconductor wafer secured to a housing by means of a bond. In any event, the housing has wire accommodating apertures or slots. These slots enable one to direct wires or leads therethrough, which leads interface with the contacts or terminals areas on the silicon diaphragm. As is explained in the patent, by routing the leads through the apertures, one obtains improved operation and better characteristics in regard to the transducing assembly. Hence, it is extremely desirable to maintain a "slotted" housing in the production of a high performance transducer.
It is also important that the edges of the diaphragm be well bonded or sealed to the housing since a simply supported diaphragm has a significantly lower sensitivity than a clamped diaphragm. In addition, as above indicated, the seal or bond must exhibit good mechanical properties as well as being leak-free over the full temperature range of the transducer.
Due to the fabrication process, a "step" is formed in the clamping region of the diaphragm which would otherwise prevent the formation of a proper seal. Therefore, due to these considerations as well as terminal placement on the diaphragm, one experiences difficulties with the electrostatic bonding technique, even though the technique can afford a good seal.
It is therefore an object of the present invention to provide an improved transducer assembly employing a slotted housing with a silicon transducer secured to the housing by means of a glass bond formed by an electrostatic process.